Potatoes are the essence of bland stolidity. They adapt to almost any cooking technique you can imagine and are happy being paired with almost any ingredient. Accounting for almost one third by weight of all the vegetables eaten in America and available year-round, they are so common we might well take them for granted. But that’s all a facade. Dig just below the surface, and you’ll find that these seemingly placid vegetables are among the strangest members of the plant kingdom.
Potatoes are members of the Solanaceae family, relatives of tomatoes, eggplants, chiles and the (really) deadly nightshade. They are tubers, which are part of the root system. But whereas true roots reach out through the soil to collect nutrients, tubers swell up to store the food the plant has collected. Tubers reproduce asexually. As every elementary school science student has learned, if you cut a potato into pieces and sow them in the ground, each piece will grow a plant exactly like the one you started with (they are true clones). Potatoes can also reproduce sexually—they have pretty white flowers that when pollinated will develop a fruit that looks like a teeny, very seedy tomato. But successful pollination occurs only very rarely, and the results are genetically haphazard—plant a potato seed, and who knows what might happen.
Potatoes are extremely demanding about their growing conditions, too. They need rich earth and are among the most nutrient-hungry of all food plants. The soil must be deep and fine; any clods will deform the spuds, so farmers are careful not to walk where potatoes are growing for fear of compacting the dirt. Potatoes also need a lot of water—about an inch per week is perfect—but the land must be well drained; soggy earth encourages rot and all kinds of fungal nastiness. Potatoes are susceptible to an amazing array of pests and diseases: early and late blights, potato beetles, mosaic virus, leafhoppers, aphids and various wilts and rots that can destroy entire fields within days. It was late blight, Phytophthora infestans, that devastated the Irish potato harvest in the late 1840s, killing more than a million people and creating a wholesale redistribution of population around the world. (A new, fungicide-resistant mutation of P. infestans has cropped up in several American growing areas.)
To help fight off this onslaught of pests and diseases, potatoes have developed their own system of self-defense. When the plant feels weak and threatened, it begins to pump up production of very potent poisons called glycoalkaloids. These are always present at low levels in the tuber and at very high levels in the leaves, flowers and stems—the most vulnerable parts of the plant. If the temperature gets too cool, if the ground gets too wet, if there aren’t enough nutrients available, if pests or diseases attack the plant or if the tuber is bruised or damaged in any way, the plant begins to step up its production of glycoalkaloids.
Even after harvest, exposure to light or heat can spur glycoalkaloid production. That’s why some people warn against eating potatoes that have begun to turn green on the skin. The color is caused by the development of chlorophyll rather than by the poisons themselves, but the two usually occur simultaneously. Sprouting potatoes are much higher in glycoalkaloids. Extremely rare cases of “potato poisoning” do occur, almost always the result of people eating potatoes that are spoiled or drinking tea made from potato leaves.
In most potatoes, the concentrations of glycoalkaloids are extremely low, less than 1,000 parts per million. Since the glycoalkaloids are concentrated just under the skin, you can eliminate most of them by peeling deeply. In fact, in small doses these poisons are one of the things that make potatoes so delicious, contributing a slight bitterness that nicely complements their natural sweetness and earthiness.
Potatoes grow in several stages. First, seed potatoes (pieces of potatoes) are planted. They produce a green plant above the ground. When the plant reaches a certain stage of maturity, rather than produce more foliage, it begins to pump nutrients below the ground to the tubers. Since the potato is actually a perennial (although it’s farmed as an annual), the tuber stores the nutrients in the form of starches and sugars so that the plant can last the winter. Gradually, the tubers swell and grow, increasing in size. One potato plant can produce as many as a dozen potatoes, with a wide range of sizes. When the tubers have absorbed as many nutrients as they can hold, the green plant begins to wilt and die. This “lay down” signals the farmer that it is time to begin the harvest.
What happens after the harvest is almost as important as what occurs before. Although a very few potatoes are sold immediately after digging, most go into storage. Those that go straight to market are true “new” potatoes, no matter the variety or size. (The sign of a real new potato is a peel that rubs off with a thumbnail.) These potatoes are usually very moist, with a complex flavor due in no small part to their being naturally higher in glycoalkaloids. The potatoes that go to curing rooms are stored at between 60 and 70 degrees with high humidity for ten days to two weeks.
Curing helps heal whatever cuts or bruises the potato might have suffered during harvest and encourages the formation of a tough skin that will protect it during storage and shipping. Gradually, the temperature in the storage room is lowered. Although all potatoes are at their best fairly soon after harvest, they can be stored for up to ten months if the conditions are carefully maintained—between 35 and 40 degrees and high humidity.
Temperature is critical. Potatoes destined for the fresh market are kept at the bottom end of that range. Those that will be fried for chips must be held at the warmer end, or even higher. That’s because chilled potatoes begin to convert starch into sugar, which will scorch when fried. The same thing can happen at home. Potatoes that have been refrigerated can taste downright sweet. When potatoes do sweeten in the refrigerator, a couple of weeks of “tempering” at slightly higher temperatures can restore some of the flavor balance.
To avoid even partial repeats of disasters such as the Irish potato famine, potato breeders are constantly working to develop new varieties with increased resistance to the spud’s many enemies. The earliest European potatoes were based on varieties that originated in the Peruvian and Bolivian Andes. As the blight showed, these were very susceptible to disease. In the late nineteenth century, breeders began experimenting with Chilean varieties, which turned out to be much more resistant. This work is ongoing, with breeders today focusing on wild Mexican varieties. Russet Burbank—the gold standard for baking potatoes—traces its family tree back to the late-nineteenth-century California plant breeder Luther Burbank. Originally bred by Burbank as a high-moisture, smooth-skinned variety, the potato we know today was an accident—a genetic mutation found and then popularized by Colorado potato farmer Lou Sweet beginning around 1910.
Whereas the first half of the twentieth century was devoted to boiling, baking and mashing potatoes, the second half was all about chips and fries. Early on, the commercial production of processed potatoes was so limited that government statisticians didn’t even include them as a separate category until the 1960s. By 1970 fresh and processed potatoes were grown in roughly equal amounts, and then fast food took off. In 1999 Americans ate 63 pounds of frozen potatoes per person (mostly French fries), compared to 50 pounds of fresh.
Although we are not eating as many fresh potatoes as we did in 1950, we are certainly eating more interesting ones. Today careful shoppers can choose among hundreds of varieties—including French fingerlings and German Butterballs, Red La Sodas, Purple Peruvians and Yukon Golds—with more being added all the time.
How do you decide which ones to use in which ways? The answer comes down to how much starch a potato contains—the higher the amount, the drier and fluffier the texture of the cooked potato. At the very top end of the scale are the russet, or baking, potatoes, such as Russet Burbank. When they are cooked, their starch cells swell and separate. At the opposite end of the scale are dense, moist potatoes such as the Red La Soda. The starch cells in these potatoes tend to stick together, even after cooking, giving their flesh a waxy texture that is perfect for dishes such as soups and potato salads, where you want the chunks of potato to remain intact and not fall apart. In the great middle are potatoes that are lumped under the nearly useless term “all-purpose.” In truth, these potatoes demand more careful handling than those at either extreme. When baked in the traditional way, they don’t have the creamy texture of true baking potatoes. They are better roasted, because they’ll hold their shape and form a nice crust on their cut surfaces. Although you can use them in potato salads, you must stir very carefully to avoid turning them into mush. They are perfect for grating into potato pancakes or hash browns and are unbeatable in gratins.
Which kind of potato you choose for mashing depends on what kind of mashed potatoes you like. For a very French, very elegant puree, one that is lighter than air but rich with butter, use a baking potato. Their light and fluffy cooked texture easily forms a silky smooth liaison with the fat. People who prefer their mashed potatoes on the chunky side should use all-purpose potatoes, since their closely packed starch cells will hold together even after you beat in butter and cream.
Care is required in mashing potatoes. The starch cells in the cooked flesh are delicate; if handled roughly, they will break, and you will wind up with something akin to wallpaper paste. A potato ricer or a food mill will give the finest texture; use a potato masher or a big fork if you like your potatoes a little chunkier. Use a folding motion and a spatula or whisk to beat in the butter and hot cream or milk. The amount of butter and whether or not you choose cream or milk is up to you. The famed French chef Joël Robuchon takes the prize for the richest mashed potatoes—nearly half spuds and half butter. Escoffier, on the other hand, used only about 10 percent butter. Let your taste be your guide.
WHERE THEY’RE GROWN: Potatoes are grown all over the United States and in just about every season, although the fall harvest centered in the Pacific Northwest dominates. Winter potatoes are grown in California and Florida, and spring potatoes are harvested primarily in Florida, North Carolina and California.
HOW TO CHOOSE: Potatoes should be firm and well filled out. Avoid any that are sprouting or show signs of greening and avoid stores that sell them. Avoid potatoes that show nicks and cuts; that’s where decay starts. Don’t worry about a little dirt in fact, the best potatoes often have a little earth sticking to their skins.
HOW TO STORE: Keep potatoes in a cool, dark place. Warm temperatures encourage sprouting and shriveling. Avoid direct sunlight, which encourages the development of substances in the potato that can make you sick. Don’t keep potatoes in the refrigerator, or they will turn sweet.
HOW TO PREPARE: To peel or not to peel? In most cases, there is no right answer. Nutrients are lost in peeling, but they are of negligible value. If a potato is greening, it should be peeled deeply. Other than that, it’s up to your taste. Many potatoes have colorful peels, so it’s good to leave them on, especially if you’re steaming them or adding them to a clear soup. Scrub unpeeled potatoes with a stiff brush to remove any soil.
ONE SIMPLE DISH: Buy an assortment of waxy potatoes of various shapes and colors, preferably new ones. Cut them so they are of roughly similar size. Steam them just until tender, about 15 minutes. While the potatoes are cooking, beat together some room-temperature butter, minced shallots and chopped herbs of your choosing in a bowl. As soon as the potatoes are done, drain, add them to the bowl and stir gently with a spatula. The butter will emulsify with the little bit of water left on the potatoes and form a sauce that will adhere lightly. Sprinkle with fleur de sel and serve immediately.